1) Field of the Invention
The present invention relates to an optical scanner and an image forming apparatus.
2) Description of the Related Art
An optical scanner may employ a single deflector to optically scan plural target surfaces. The optical scanner is used in an image forming device to form a color image as is known in the art. When such optical scan mode is applied to a color image forming device, it is not required to use the deflector more than one. In this case, the number of plural light sources required is equal to or more than that of the target surfaces (the number equal to that of the target surfaces in a single beam scan mode, and the number equal to or more than that of the target surfaces in a multi-beam scan mode). In addition, as the light sources are arranged separately, the number of components for light source arrangement increases. When environmental fluctuations and the like cause relative variations in optical scanning with beams from the light sources, the variations raise a phenomenon called “out-of-color registration”, which deteriorates the image quality in a color image to be formed.
Proposed as a configuration of the above optical scanner is a “system that passes plural beams traveling toward different target surface” through a scanning lens proximate to the deflector (see Japanese Patent Application Laid-Open No. 2001-4948).
This optical scanner can reduce the out-of-color registration due to the environmental variation because plural beams traveling toward different target surfaces pass through the same scanning lens. In this case, however, plural beams traveling toward the deflector have no open angle in a deflecting rotation plane. Therefore, it is required to locate an additional optical path deflector such as a prism before the deflector, which increases the number of components and easily invites cost-elevation. The optical path deflector, for example, the prism easily causes a deteriorated optical characteristic and a reduced utilization efficiency of light.
In the conventional color image forming device, “photodetectors operative to receive deflected beams” for use in timing control of optical scanning are arranged individually as corresponding to different target surfaces. This arrangement invites an increase in the number of components and cost-elevation. In addition, if relative arrangements of the photodetectors fluctuate due to environmental variations, initial positions of optical scanning in the target surfaces may be changed relatively to cause the out-of-color registration in a color image to be formed.
In recent years, for achievement of color digital copiers and color laser printers with higher recording speeds, different colored-images are formed on plural target surfaces. These images are then sequentially transferred onto a recording medium to complete a color image. Such devices have been known widely as so-called “tandem type color image forming devices”.
Proposed as such the tandem type color image forming device is an optical scanner that includes a single deflector sandwiched between scanning optical systems arranged at both sides thereof to optically scan four photosensitive members at the same time (see Japanese Patent Application Laid-Open No. 2002-90672).
The higher the image quality of color images to be formed, the more the reduction of light spot diameters proceeds. In order to reduce a light spot diameter, another proposal is given to a scanning lens. This scanning lens employs a special toric surface, which has the varying radius of a sub scan curvature from the optical axis of the lens surface to peripheries in the main scan direction (see Japanese Patent Application Laid-Open No. 2001-324689).
In the tandem type color image forming device disclosed in Japanese Patent Application Laid-Open No. 2002-90672, beams from a plurality of light sources enter a light deflector while having an “open angle” in a deflecting rotation plane toward the light deflector. Therefore, they have different average incident angles to the optical axis of the scanning optical system, resulting in a sag-effected deterioration of optical characteristics, particularly curvature of the image plane in the sub scan direction, which makes it difficult to reduce light spot diameters.